In amplification of ultrashort optical pulses (e.g., femtosecond, i.e. 1015 second range pulse width), the optical peak intensities that occur can become very high, so that detrimental nonlinear pulse distortion or even destruction of the gain medium or of some other optical element may occur. This can be prevented by employing a method known as chirped-pulse amplification (CPA).
In CPA, before passing through the amplifier medium of the amplifier, the pulses are “chirped” to provide temporary stretching to a much longer pulse duration by a strongly dispersive element (referred to as the pulse stretcher, e.g., a grating pair used with a telescope or a long fiber). As known in the art, an ultrashort light pulse necessarily has a broad spectral bandwidth by virtue of the uncertainty principle.
The pulse stretcher possesses a wavelength-dependent optical path length, so that different wavelength components have differing transit times through the pulse stretcher. Thus, an ultrashort light pulse becomes spread in time, often by a factor of 10,000 times or more. For example, a 10 fs light pulse can emerge from a pulse stretcher with a time duration of >100 picoseconds. Stretching thus reduces the peak pulse power to a level where the above-mentioned detrimental effects in the gain medium are avoided, or at least minimized. The stretched pulse still possesses the frequency spectrum necessary for a much-shorter pulse.
After the optical pulses exit the gain medium, a dispersive compressor is used, i.e., an element with opposite dispersion (typically a grating pair) that has wavelength-dependent optical path length that largely “undoes” the effect of the stretcher, regaining an ultrashort pulse having a pulse duration similar to the input pulse duration with dramatically higher peak power than was present in the amplifier system itself.
One strongly dispersive element is a Bragg grating (BG). Unlike a surface grating which is a 2 dimensional gating that is analogous to a metal mirror, a Bragg grating is a three dimensional grating, often having an interaction length that can be 1 cm or more. BGs can have a constant grating period, or a variable grating period. BGs having a constant grating period wavelength can be used for wavelength locking, for example. The BG can be embodied as a volume BG, a fiber BG, or a waveguide BG.
In the case of a variable grating period, the BG is referred to as a chirped BG. Used in a CPA application, a single chirped BG can replace both the pulse stretcher and compressor, providing stretching and compressing. The path of the pulse through the chirped BG's grating determines whether the pulse is stretched or compressed. The single chirped BG embodiment provides the laser system a significant reduction in volume, less weight, easier alignment and maintenance, lower sensitivity to vibration, and significant cost reduction.
FIG. 1A shows a depiction of a conventional single chirped BG 100 comprising an optically transparent substrate 110 that is optically transparent within a predetermined wavelength or wavelength range. Chirped BG 100 has a length dimension that the optical beam traverses and interacts therewith, shown as L, and a transverse (cross sectional area defining) dimension shown as A. The chirped BG 100 may be fabricated using a photorefractive glass that has an altered refractive index in areas that have been exposed to UV light. FIG. 1B shows the spatial coordinate when the period of the BG 100 varies monotonically along the spatial axis as shown in FIG. 1A.
BGs are available in relatively large cross-sectional sizes as compared to the other BGs (e.g., fiber, waveguide), such as a maximum of about 5 mm×5 mm. This maximum cross sectional area is limited by material defects. However, since the peak power becomes very high during compression, the power of the chirped BG comprising laser system is limited by the chirped BG to about <0.5 mJ/pulse to avoid optical damage. Similarly, for high power laser applications that involve constant period BGs, such as for wavelength locking, the BG can limit the power that can be used in the laser system.